B2.1 – Cell Structure

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1. 1. State that living organisms are made of cells.

Living organisms are made of cells.

Cells are the basic unit of life.

 

2. Identify and describe the structure of a plant cell (palisade cell) and an animal cell (liver cell), as seen under a light microscope.

Plant cell:

The cell wall is the strong outermost layer of a plant cell. It is made of cellulose.

The cell membrane is a bilayer (two layer) membrane that covers the contents of the cell.

Cytoplasm is the jelly-like substance that fills the cell. It is made of 80% water.

The nucleus is known as the ‘brain’ of the cell, because this is where all the genetic material is contained, and it’s the genetic material that tells the cell what to do.

The nucleus contains chromatin – the substance that chromosomes are made of.

Inside the nucleus is the nucleolus, which is a dense spherical region in the nucleus.

The endoplasmic reticulum (ER) is a structure which shares its membrane with the nucleus. The membrane is continuous. There are two types of ERs – rough ER (named so because it is dotted with ribosomes) and smooth ER (not dotted with ribosomes).

Ribosomes are the tiny spherical structures found in the cell cytoplasm and on the Rough ER.

Golgi apparatus or Golgi complexes or Golgi bodies are long sausage-like structures found in the cell cytoplasm. Transport and secretory vesicles are often found near them.

Mitochondria are small structures in the cytoplasm with inwards foldings or finger-like projections.

The chloroplast is a structure containing chlorophyll.

The vacuole is a double membranous structure containing cell sap. Cell sap is mainly a sugar solution.

Under a light microscope, a plant cell looks like this:

 

Animal Cell

The nuclear envelope is the membrane of the nucleus and nuclear pores are the holes in this membrane.
Centrioles are bundles of microtubules and are always found in pairs except during cell division. During cell division, they split.

Lysosomes are membranous structures containing digestive enzymes.

An animal cell under a light microscope looks like this:

 

3. Describe the differences in structure between typical animal and plant cells.

 

4. Relate the structures seen under the light microscope in the plant cell and in the animal cell to their functions.

The outermost layer of plant cells is the cell wall. It’s made of cellulose like I said before, and its main function is to maintain the cell shape.

Both animal and plant cells have a cell membrane – this separates the contents of the cell from its outside environment. The cell membrane also controls what goes in and out of the cell – like border control.

The cytoplasm of the cell is made largely of water and it helps to suspend the cell organelles in space.

The nucleus of the cell is its brain – all the genetic material is here. Genes contain the code for carrying out the different functions of the cell.

The nucleolus is a dense region in the nucleus with the primary function of synthesising ribosomes (which are used to produce proteins) and RNA.

The smooth ER is used to produce lipids (fats) and the rough ER is involved in protein synthesis.

The Golgi apparatus is used to transport substances: once a lipid or a protein has been produced by the ER, it pinches off a piece of the ER membrane, so that it is now surrounded by a membrane. It forms a transport vesicle. This transport vesicle travels to the Golgi apparatus. The membrane fuses with the membrane of the Golgi apparatus, and the protein or lipid pinches of a piece off a piece of the Golgi apparatus membrane to form a vesicle. If this vesicle is to be transported outside the cell, it is known as a secretory vesicle. If it is to be transported to cell organelles within the cell or to remain in the cell cytoplasm, the vesicle is known as a transport vesicle.

Mitochondria are the powerhouses of the cell – they produce ATP (adenosine triphosphate) which is necessary for respiration.

The chloroplast is a double membranous structure containing the green pigment chlorophyll. Chlorophyll absorbs energy from sunlight and stores it as chemical energy. This energy is used to synthesise starch – the food of plants.

The vacuole in plants is used for cell turgidity – when it is full of water, it pushes out on the cell, so the cell assumes its proper shape. The vacuole also contains enzymes and other substances that are useful to the cell.

Centrioles are used to organise the cell during cell division.

Lysosomes are known as suicide bags. They are membranous structures containing digestive enzymes. At the time of cell death, these membranes break open to release these enzymes, and these enzymes digest the cell.

1. 1. 5. Relate the structure of the following to their functions:
      • Red blood cells – transport

Red blood cells are used to transport oxygen to the muscles, via the blood. They have a biconcave disc shape (like a disk, where the bottom and the top dip into the middle). This gives it a lot of surface area for fast oxygen absorption and allows it to flow easily through the blood vessels. They have haemoglobin – the protein that combines with oxygen to form oxyhaemoglobin – which allows it to transport oxygen. They have no nucleus, which gives them more space for haemoglobin. RBCs also have a thin membrane, allowing easy diffusion of oxygen.

1. 1. • Root hair cells – absorption

Root hair cells have an elongated structure called the root hair – this gives them a lot of surface area for easy absorption of water and mineral ions. Root hair cells have a tough wall, which allows them to reach deep underground to where the water and ions are. The salt concentration in their cell sap is higher than in the surrounding soil water, so water can diffuse in by osmosis, down the concentration gradient. Root hair cells have a semi-permeable membrane too – this permits water and mineral ions to enter but not leave.
(We’ll learn about osmosis and the concentration gradient later)

 

1. 1. 6. Calculate the magnification and size of biological specimens using millimetres as units.

We use a formula, where

M=I/A

Here, M is magnification, I is image size (the size of the specimen on the image) and A is actual size (the actual size of the specimen).

 

 

 

Notes Submitted by Sarah.

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